Catalyst for treating exhaust gas

ABSTRACT

Provided is a method for reducing N 2 O emissions in an exhaust gas comprising contacting an exhaust gas containing NH 3  and an inlet NO concentration with an SCR catalyst composition containing small pore zeolite having an SAR of about 3 to about 15 and having about 1-5 wt. % of an exchanged transition metal.

BACKGROUND

1. Field of Invention The present invention relates to catalysts,articles, and methods for treating combustion exhaust gas.

2. Description of Related Art

Combustion of hydrocarbon-based fuel in engines produces exhaust gasthat contains, in large part, relatively benign nitrogen (N₂), watervapor (H₂O), and carbon dioxide (CO₂). But the exhaust gases alsocontain, in relatively small part, noxious and/or toxic substances, suchas carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC)from un-burnt fuel, nitrogen oxides (NO_(x)) from excessive combustiontemperatures, and particulate matter (mostly soot). To mitigate theenvironmental impact of flue and exhaust gas released into theatmosphere, it is desirable to eliminate or reduce the amount of theundesirable components, preferably by a process that, in turn, does notgenerate other noxious or toxic substances.

Typically, exhaust gases from lean burn gas engines have a net oxidizingeffect due to the high proportion of oxygen that is provided to ensureadequate combustion of the hydrocarbon fuel. In such gases, one of themost burdensome components to remove is NO_(x), which includes nitricoxide (NO), nitrogen dioxide (NO₂), and nitrous oxide (N₂O). Thereduction of NO_(x) to N₂ is particularly problematic because theexhaust gas contains enough oxygen to favor oxidative reactions insteadof reduction. Notwithstanding, NO can be reduced by a process commonlyknown as Selective Catalytic Reduction (SCR). An SCR process involvesthe conversion of NO_(x), in the presence of a catalyst and with the aidof a reducing agent, such as ammonia, into elemental nitrogen (N₂) andwater. In an SCR process, a gaseous reductant such as ammonia is addedto an exhaust gas stream prior to contacting the exhaust gas with theSCR catalyst. The reductant is absorbed onto the catalyst and the NOreduction reaction takes place as the gases pass through or over thecatalyzed substrate. The chemical equation for stoichiometric SCRreactions using ammonia is:

4NO+4NH₃+O₂→4N₂+6H₂O

2NO₂+4NH₃+P₂→3N₂+6H₂O

NO+NO₂+2NH₃→2N₂+3H₂O

Zeolites having an exchanged transition metal are known to be useful asSCR catalysts. Conventional small pore zeolites exchanged with copperare particularly useful in achieving high NO_(x) conversion at lowtemperatures. However, the interaction of NH₃ with NO absorbed onto thetransition metal of an exchanged zeolite can lead to an undesirable sidereaction that produces N₂O. This N₂O is particularly problematic toremove from the exhaust stream. Accordingly, there remains a need forimproved methods that result in a high conversion of NO_(x). The presentinvention satisfies this need amongst others.

SUMMARY OF THE INVENTION

Surprisingly, it has been found that metal-exchanged zeolites having asmall pore structure and a silica-to-alumina ratio (SAR) of about 3 toabout 15 result in a decrease production of N₂O compared to similarmetal-exchanged zeolites having a higher SAR value. Accordingly, thepresent invention provides improved catalytic performance inapplications such as selective catalytic reduction (SCR) of NO_(x).

Accordingly, in one aspect of the invention, provided is a method forreducing N₂O emissions in an exhaust gas comprising contacting anexhaust gas containing NH₃ and an inlet NO concentration with an SCRcatalyst composition containing small pore zeolite having an SAR ofabout 3 to about 15 and having about 1-5 wt. % of an exchangedtransition metal to produce a purified gas containing an outlet NOconcentration and an outlet N₂O concentration, wherein (a) the inlet NOconcentration and outlet NO concentration have a relative ratio of>about4, and (b) the inlet NO concentration and outlet N₂O concentration havea relative ratio of>about 50. As used herein, the terms “inlet” and“outlet” exhaust gas (or relative component concentrations) mean theexhaust gas (or relative component concentrations) immediately upstreamand downstream, respectively, of the SCR and/or ASC filter. The term“immediately upstream” and “immediately downstream” mean that theexhaust gas prior to and subsequent to, respectively, the SCR and/or ASCcatalyst without any intervening catalyst operations that would decreasethe N₂O concentration of the purified exhaust gas.

According to another aspect of the invention, provided is a system fortreating an exhaust gas comprising, in series and in fluidcommunication, a diesel oxidation catalyst, a source of nitrogen-basedreductant, and an SCR catalyst, wherein the SCR catalyst comprises asmall pore zeolite having an SAR of about 3 to about 15 and having about1-5 wt. % of an exchanged transition metal, and wherein the SCR catalystis coated on a honeycomb wall-flow filter or flow-through monolith or isan extruded honeycomb body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows NO_(x) conversion (solid lines) and N₂O generation (dashedlines) during steady state data evaluation of certain embodiments of theinvention and comparative examples at a GHSV of 50,000 h⁻¹ and anammonia to NO_(x) ratio of 1.

FIG. 2 shows a schematic diagram of an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In a certain aspect, the invention is directed to a method for improvingenvironmental air quality, particularly for improving exhaust gasemissions generated by power plants, gas turbines, lean burn internalcombustion engines, and the like. Exhaust gas emissions are improved, atleast in part, by reducing NO_(x) and N₂O concentrations over a broadoperational temperature range.

The method for reducing N₂O emissions in an exhaust gas can comprise thesteps of contacting an exhaust gas containing NH₃ and an inlet NOconcentration with an SCR catalyst composition containing small porezeolite having an SAR of about 3 to about 15 and having about 1-5 wt. %of an exchanged transition metal to produce a purified gas containing alow outlet NO concentration and a low outlet N₂O concentration, wherein(a) the inlet NO concentration and outlet NO concentration have arelative ratio of greater than about 4, and (b) the inlet NOconcentration and outlet N₂O concentration have a relative ratio ofgreater than about 50. According to the invention, the contacting occursat a temperature of less than about 350° C., for example about 150-350°C. or about 200-300° C.; or at a temperature less than about 700° C.and/or at a temperature greater than about 350° C. or greater than about450° C., for example about 350-700° C., about 350-600° C., or about450-550° C.

In certain embodiments, the exhaust gas has a relative high NO₂:NOratio, for example, at least about 4:1, at least about 10:1, or at leastabout 20:1. In certain embodiments, the inlet NO₂ concentration andoutlet NO₂ concentration of the exhaust gas have a relative ratio ofgreater than about 4, for example at least about 5, at least about 10,or at least about 20, and (b) the inlet NO₂ concentration and outlet N₂Oconcentration have a relative ratio of greater than about 50, forexample at least about 100 or at least about 200.

Zeolites of the present invention are crystalline or quasi-crystallinealuminosilicates which are constructed of repeating SiO₄ and AlO₄tetrahedral units linked together, for example in rings, to formframeworks having regular intra-crystalline cavities and channels ofmolecular dimensions. The specific arrangement of tetrahedral unitsgives rise to the zeolite framework, and by convention, each uniqueframework is assigned a unique three-letter code (e.g., “AEI”) by theInternational Zeolite Association (IZA).

Particularly useful zeolites to the present invention are small porezeolites. As used herein, the term “small pore zeolite” means a zeoliteframework having a maximum ring size of eight tetrahedral atoms. In someexamples, the small pore zeolite for use in the present invention have apore size in at least one dimension of less than 4.3 Å. In oneembodiment, the small pore zeolite has a framework selected from thegroup of consisting of: ACO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD,ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, GIS, GOO, IHW, ITE, ITW,LEV, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SIV, THO,TSC, UEI, UFI, VNI, YUG and ZON. Preferred zeolite frameworks areselected from AEI, AFT, AFX, CHA, DDR, ERI, LEV, KFI, RHO, and UEI. Forcertain applications, preferred zeolite frameworks are selected fromAEI, AFT, and AFX, particularly AEI. In certain application, a preferredzeolite framework is CHA. In certain applications, an ERI framework ispreferred. In certain embodiments, the zeolite is essentially free ofCHA frameworks. In certain embodiments, the zeolite is essentially freeof AEI frameworks. In certain embodiments, the zeolite is essentiallyfree of ERI frameworks. Particular zeolites that are useful for thepresent invention include SSZ-39, Mu-10, SSZ-16, SSZ-13, Sigma-1,ZSM-34, NU-3, ZK-5, and MU-18.

Preferably, the primary crystalline phase of the zeolite is constructedof one or more small pore frameworks, although other aluminosilicatecrystalline phases may also be present. Preferably, the primarycrystalline phase comprises at least about 90 weight percent, morepreferably at least about 95 weight percent, and even more preferably atleast about 98 or at least about 99 weight percent small pore zeoliteframework, based on the total amount of zeolite in the material.

Preferably, a majority of the zeolite structure is constructed ofalumina and silica. The zeolite may include framework metals other thanaluminum (i.e., metal-substituted zeolites). As used herein, the term“metal substituted” with respect to a zeolite means a zeolite frameworkin which one or more aluminum or silicon framework atoms has beenreplaced by the substituting metal. In contrast, the term “metalexchanged” means a zeolite in which one or more ionic species associatedwith the zeolite (e.g., H⁺, NH4⁺, Na⁺, etc.) has been replaced by ametal (e.g., a metal ion or free metal, such as metal oxide), whereinthe metal is not incorporated as a zeolite framework atom (e.g.,T-atom), but instead is incorporated into the molecular pores or on theexternal surface of the zeolite framework.

Preferably, the zeolite is free or essentially free of phosphorous.Thus, the term “zeolite” as used herein, does not encompasssilicoaluminophosphate molecular sieves (SAPOs) or aluminophosphates(AIPOs).

Preferred zeolites have a silica-to-alumina ratio of about 3 to about15, such as about 3 to about 5, about 5 to about 10, about 10 to about15, about 3 to about 9, about 5 to about 7, or about 9 to about 14. Aminimum SAR is required to achieve an adequate thermostability.Surprisingly, a maximum SAR value is necessary to prevent or limit theformation of N₂O.

The silica-to-alumina ratio of zeolites may be determined byconventional analysis. This ratio is meant to represent, as closely aspossible, the ratio in the rigid atomic framework of the zeolite crystaland to exclude silicon or aluminum in the binder or in cationic or otherform within the channels. Since it may be difficult to directly measurethe silica to alumina ratio of zeolite after it has been combined with abinder material, particularly an alumina binder, these silica-to-aluminaratios are expressed in terms of the SAR of the zeolite per se, i.e.,prior to the combination of the zeolite with the other catalystcomponents.

The catalyst composition comprises at least one transition metaldisposed on and/or within the zeolite material as extra-framework metals(also referred to herein as a metal-promoted zeolite). As used herein,an “extra-framework metal” is one that resides within the molecularsieve and/or on at least a portion of the molecular sieve surface,preferably as an ionic species, does not include aluminum, and does notinclude atoms constituting the framework of the molecular sieve.Preferably, the presence of the transition metal facilitates thetreatment of exhaust gases, such as exhaust gas from a diesel engine,including processes such as NO_(x) reduction, NH₃ oxidation, and NO_(x)storage, while also suppressing the formation of N₂O.

The transition metal may be any of the recognized catalytically activemetals that are used in the catalyst industry to form metal-exchangedzeolites, particularly those metals that are known to be catalyticallyactive for treating exhaust gases derived from a combustion process.Particularly preferred are metals useful in NO_(x) reduction and storageprocesses. Transition metal should be broadly interpreted andspecifically includes copper, nickel, zinc, iron, tungsten, molybdenum,cobalt, titanium, zirconium, manganese, chromium, vanadium, niobium, aswell as tin, bismuth, and antimony; platinum group metals, such asruthenium, rhodium, palladium, indium, platinum, and precious metalssuch as gold and silver. Preferred transition metals are base metals,and preferred base metals include those selected from the groupconsisting of chromium, manganese, iron, cobalt, nickel, and copper, andmixtures thereof. In a preferred embodiment, at least one of thetransition metals is copper. Other preferred transition metals includeiron, particularly in combination with copper.

In certain embodiments, the transition metal is present in the zeolitematerial at a concentration of about 0.1 to about 10 weight percent (wt%) based on the total weight of the zeolite, for example from about 0.5wt % to about 5 wt %, from about 0.5 to about 1 wt %, from about 1 toabout 5 wt %, about 2.5 wt % to about 3.5 wt %, and about 3 wt % toabout 3.5 wt %. For embodiments which utilize copper, the concentrationof these transition metals in the zeolite material is preferably about0.5 to about 5 weight percent, more preferably about 2.5 to about 3.5weight percent.

In certain embodiments, the transition metal, such as copper, is presentin an amount from about 80 to about 120 g/ft³ of zeolite or washcoatloading, including for example about 85 to about 95 g/ft³, or about 90to about 95 g/ft³.

In certain embodiments, the transition metal is present in an amountrelative to the amount of aluminum in the zeolite, namely the frameworkaluminum. As used herein, the transition metal:aluminum (M:Al) ratio isbased on the relative molar amount of transition metal to molarframework Al in the corresponding zeolite. In certain embodiments, thecatalyst material has a M:Al ratio of about 0.1 to about 1.0, preferablyabout 0.2 to about 0.5. An M:Al ratio of about 0.2 to about 0.5 isparticularly useful where M is copper, and more particularly where M iscopper and the SAR of the zeolite is about 10-15.

Preferably, the metal is highly dispersed within the zeolite crystals,preferably without a high temperature treatment of the metal loadedzeolite. For embodiments which utilize copper, the copper loading ispreferably fully ion exchanges and/or is preferably less than can beaccommodated by the exchange sites of the zeolite support. Preferably,the catalyst is free of substantially free of bulk copper oxide, free orsubstantially free of species of copper on external zeolite crystalsurfaces, and/or free or substantially free of copper metal clusters asmeasured by temperature programmed reduction (TPR) analysis and/orUV-vis analysis.

In one example, a metal-exchanged zeolite is created by blending thezeolite, for example a H-form zeolite or an NH₄-form zeolite, into asolution containing soluble precursors of the catalytically activemetal(s). The pH of the solution may be adjusted to induce precipitationof the catalytically active metal cations onto or within the zeolitestructure (but not including the zeolite framework). For example, in apreferred embodiment, a zeolite material is immersed in a solutioncontaining copper nitrate or copper acetate for a time sufficient toallow incorporation of the catalytically active copper cations into themolecular sieve structure by ion exchange. Un-exchanged copper ions areprecipitated out. Depending on the application, a portion of theun-exchanged ions can remain in the molecular sieve material as freecopper. The metal-exchanged zeolite may then be washed, dried, andcalcined. The calcined material may include a certain percentage ofcopper as copper oxide residing on the surface of the zeolite or withinthe zeolite cavities.

Generally, ion exchange of the catalytic metal cation into or on thezeolite may be carried out at room temperature or at a temperature up toabout 80° C. over a period of about 1 to 24 hours at a pH of about 7.The resulting catalytic molecular sieve material is preferably dried atabout 100 to 120° C. overnight and calcined at a temperature of at leastabout 500° C.

In certain embodiments, the catalyst composition comprises thecombination of at least one transition metal and at least one alkali oralkaline earth metal, wherein the transition metal(s) and alkali oralkaline earth metal(s) are disposed on or within the zeolite material.The alkali or alkaline earth metal can be selected from sodium,potassium, rubidium, cesium, magnesium, calcium, strontium, barium, orsome combination thereof. As used here, the phrase “alkali or alkalineearth metal” does not mean the alkali metals and alkaline earth metalsare used in the alternative, but instead that one or more alkali metalscan be used alone or in combination with one or more alkaline earthmetals and that one or more alkaline earth metals can be used alone orin combination with one or more alkali metals.

In certain embodiments, alkali metals are preferred. In certainembodiments, alkaline earth metals are preferred. Preferred alkali oralkaline earth metals include calcium, potassium, and combinationsthereof. In certain embodiments, the catalyst composition is essentiallyfree of magnesium and/or barium. In certain embodiments, the catalyst isessentially free of any alkali or alkaline earth metal except calciumand potassium. In certain embodiments, the catalyst is essentially freeof any alkali or alkaline earth metal except calcium. And in certainother embodiments, the catalyst is essentially free of any alkali oralkaline earth metal except potassium. As used herein, the term“essentially free” means that the material does not have an appreciableamount of the particular metal. That is, the particular metal is notpresent in amount that would affect the basic physical and/or chemicalproperties of the material, particularly with respect to the material'scapacity to selectively reduce or store NO_(x).

In certain embodiments, the zeolite material has an alkali content ofless than 3 weight percent, more preferably less than 1 weight percent,and even more preferably less than 0.1 weight percent.

In certain embodiments, the alkali and/or alkaline earth metal(collectively A_(M)) is present in the zeolite material in an amountrelative to the amount of transition metal (M) in the zeolite.Preferably, the M and A_(M) are present, respectively, in a molar ratioof about 15:1 to about 1:1, for example about 10:1 to about 2:1, about10:1 to about 3:1, or about 6:1 to about 4:1, particularly were M iscopper and A_(M) is calcium. In certain embodiments which include analkali and/or alkaline earth metal such as calcium, the amount of copperpresent is less than 2.5 weight percent, for example less than 2 weightpercent or less than 1 weight percent, based on the weight of thezeolite.

In certain embodiments, the relative cumulative amount of transitionmetal (M) and alkali and/or alkaline earth metal (A_(M)) is present inthe zeolite material in an amount relative to the amount of aluminum inthe zeolite, namely the framework aluminum. As used herein, the(M+A_(M)):Al ratio is based on the relative molar amounts of M+A_(M) tomolar framework Al in the corresponding zeolite. In certain embodiments,the catalyst material has a (M+A_(M)):Al ratio of not more than about0.6. In certain embodiments, the (M+A_(M)):Al ratio is not more than0.5, for example about 0.05 to about 0.5, about 0.1 to about 0.4, orabout 0.1 to about 0.2.

The transition metal and alkali/alkaline earth metal can be added to themolecular sieve via any known technique such as ion exchange,impregnation, isomorphous substitution, etc. The transition metal andthe alkali or alkaline earth metal can be added to the zeolite materialin any order (e.g., the metal can be exchanged before, after, orconcurrently with the alkali or alkaline earth metal), but preferablythe alkali or alkaline earth metal is added prior to or concurrentlywith the transition metal, particularly when the alkali earth metal iscalcium and the transition metal is copper.

In certain embodiments, the metal promoted zeolite catalysts of thepresent invention also contain a relatively large amount of cerium (Ce).In certain embodiments, the cerium concentration in the catalystmaterial is present in a concentration of at least about 1 weightpercent, based on the total weight of the zeolite. Examples of preferredconcentrations include at least about 2.5 weight percent, at least about5 weight percent, at least about 8 weight percent, at least about 10weight percent, about 1.35 to about 13.5 weight percent, about 2.7 toabout 13.5 weight percent, about 2.7 to about 8.1 weight percent, about2 to about 4 weight percent, about 2 to about 9.5 weight percent, andabout 5 to about 9.5 weight percent, based on the total weight of thezeolite. In certain embodiments, the cerium concentration in thecatalyst material is about 50 to about 550 g/ft³. Other ranges of Ceinclude: above 100 g/ft³, above 200 g/ft³, above 300 g/ft³, above 400g/ft³, above 500 g/ft³, from about 75 to about 350 g/ft³, from about 100to about 300 g/ft³, and from about 100 to about 250 g/ft³.

In certain embodiments, the concentration of Ce exceeds the theoreticalmaximum amount available for exchange on the metal-promoted zeolite.Accordingly, in some embodiments, Ce is present in more than one form,such as Ce ions, monomeric ceria, oligomeric ceria, and combinationsthereof, provided that said oligomeric ceria has a mean crystal size ofless than 5 μm, for example less than 1 μm, about 10 nm to about 1 μm,about 100 nm to about 1 μm, about 500 nm to about 1 μm, about 10 toabout 500 nm, about 100 to about 500 nm, and about 10 to about 100 nm.As used herein, the term “monomeric ceria” means CeO₂ as individualmolecules or moieties residing freely on and/or in the zeolite or weaklybonded to the zeolite. As used herein, the term “oligomeric ceria” meansnanocrystalline CeO₂ residing freely on and/or in the zeolite or weaklybonded to the zeolite.

Catalysts of the present invention are applicable for heterogeneouscatalytic reaction systems (i.e., solid catalyst in contact with a gasreactant). To improve contact surface area, mechanical stability, and/orfluid flow characteristics, the catalysts can be disposed on and/orwithin a substrate, preferably a porous substrate. In certainembodiments, a washcoat containing the catalyst is applied to an inertsubstrate, such as corrugated metal plate or a honeycomb cordieritebrick. Alternatively, the catalyst is kneaded along with othercomponents such as fillers, binders, and reinforcing agents, into anextrudable paste which is then extruded through a die to form ahoneycomb brick. Accordingly, in certain embodiments provided is acatalyst article comprising a metal-promoted zeolite catalyst describedherein coated on and/or incorporated into a substrate.

Certain aspects of the invention provide a catalytic washcoat. Thewashcoat comprising the metal promoted zeolite catalyst described hereinis preferably a solution, suspension, or slurry. Suitable coatingsinclude surface coatings, coatings that penetrate a portion of thesubstrate, coatings that permeate the substrate, or some combinationthereof.

A washcoat can also include non-catalytic components, such as fillers,binders, stabilizers, rheology modifiers, and other additives, includingone or more of alumina, silica, non-zeolite silica alumina, titania,zirconia, ceria. In certain embodiments, the catalyst composition maycomprise pore-forming agents such as graphite, cellulose, starch,polyacrylate, and polyethylene, and the like. These additionalcomponents do not necessarily catalyze the desired reaction, but insteadimprove the catalytic material's effectiveness, for example, byincreasing its operating temperature range, increasing contact surfacearea of the catalyst, increasing adherence of the catalyst to asubstrate, etc. In preferred embodiments, the washcoat loading is >0.3g/in³, such as >1.2 g/in³, >1.5 g/in³, >1.7 g/in³ or >2.00 g/in³, andpreferably <3.5 g/in³, such as <2.5 g/in³. In certain embodiments, thewashcoat is applied to a substrate in a loading of about 0.8 to 1.0g/in³, 1.0 to 1.5 g/in³, or 1.5 to 2.5 g/in³.

Two of the most common substrate designs are plate and honeycomb.Preferred substrates, particularly for mobile applications, includeflow-through monoliths having a so-called honeycomb geometry thatcomprise multiple adjacent, parallel channels that are open on both endsand generally extend from the inlet face to the outlet face of thesubstrate and result in a high-surface area-to-volume ratio. For certainapplications, the honeycomb flow-through monolith preferably has a highcell density, for example about 600 to 800 cells per square inch, and/oran average internal wall thickness of about 0.18-0.35 mm, preferablyabout 0.20-0.25 mm. For certain other applications, the honeycombflow-through monolith preferably has a low cell density of about 150-600cells per square inch, more preferably about 200-400 cells per squareinch. Preferably, the honeycomb monoliths are porous. In addition tocordierite, silicon carbide, silicon nitride, ceramic, and metal, othermaterials that can be used for the substrate include aluminum nitride,silicon nitride, aluminum titanate, α-alumina, mullite, e.g., acicularmullite, pollucite, a thermet such as Al₂OsZFe, Al₂O₃/Ni or B₄CZFe, orcomposites comprising segments of any two or more thereof. Preferredmaterials include cordierite, silicon carbide, and alumina titanate.

Plate-type catalysts have lower pressure drops and are less susceptibleto plugging and fouling than the honeycomb types, which is advantageousin high efficiency stationary applications, but plate configurations canbe much larger and more expensive. A Honeycomb configuration istypically smaller than a plate type, which is an advantage in mobileapplications, but has higher pressure drops and plug more easily. Incertain embodiments the plate substrate is constructed of metal,preferably corrugated metal.

In certain embodiments, the invention is a catalyst article made by aprocess described herein. In a particular embodiment, the catalystarticle is produced by a process that includes the steps of applying ametal-promoted zeolite composition, preferably as a washcoat, to asubstrate as a layer either before or after at least one additionallayer of another composition for treating exhaust gas has been appliedto the substrate. The one or more catalyst layers on the substrate,including the metal-promoted zeolite catalyst layer, are arranged inconsecutive layers. As used herein, the term “consecutive” with respectto catalyst layers on a substrate means that each layer is contact withits adjacent layer(s) and that the catalyst layers as a whole arearranged one on top of another on the substrate.

In certain embodiments, the metal-promoted zeolite catalyst is disposedon the substrate as a first layer and another composition, such as anoxidation catalyst, reduction catalyst, scavenging component, or NO_(x)storage component, is disposed on the substrate as a second layer. Inother embodiments, the metal-promoted zeolite catalyst is disposed onthe substrate as a second layer and another composition, such as such asan oxidation catalyst, reduction catalyst, scavenging component, orNO_(x) storage component, is disposed on the substrate as a first layer.As used herein the terms “first layer” and “second layer” are used todescribe the relative positions of catalyst layers in the catalystarticle with respect to the normal direction of exhaust gasflow-through, past, and/or over the catalyst article. Under normalexhaust gas flow conditions, exhaust gas contacts the first layer priorto contacting the second layer. In certain embodiments, the second layeris applied to an inert substrate as a bottom layer and the first layeris top layer that is applied over the second layer as a consecutiveseries of sub-layers. In such embodiments, the exhaust gas penetrates(and hence contacts) the first layer, before contacting the secondlayer, and subsequently returns through the first layer to exit thecatalyst component. In other embodiments, the first layer is a firstzone disposed on an upstream portion of the substrate and the secondlayer is disposed on the substrate as a second zone, wherein the secondzone is downstream of the first.

In another embodiment, the catalyst article is produced by a processthat includes the steps of applying a metal-promoted zeolite catalystcomposition, preferably as a washcoat, to a substrate as a first zone,and subsequently applying at least one additional composition fortreating an exhaust gas to the substrate as a second zone, wherein atleast a portion of the first zone is downstream of the second zone.Alternatively, the metal-promoted zeolite catalyst composition can beapplied to the substrate in a second zone that is downstream of a firstzone containing the additional composition. Examples of additionalcompositions include oxidation catalysts, reduction catalysts,scavenging components (e.g., for sulfur, water, etc.), or NO_(x) storagecomponents.

To reduce the amount of space required for an exhaust system, individualexhaust components in certain embodiments are designed to perform morethan one function. For example, applying an SCR catalyst to a wall-flowfilter substrate instead of a flow-through substrate serves to reducethe overall size of an exhaust treatment system by allowing onesubstrate to serve two functions, namely catalytically reducing NO_(x)concentration in the exhaust gas and mechanically removing soot from theexhaust gas. Accordingly, in certain embodiments, the substrate is ahoneycomb wall-flow filter or partial filter. Wall-flow filters aresimilar to flow-through honeycomb substrates in that they contain aplurality of adjacent, parallel channels. However, the channels offlow-through honeycomb substrates are open at both ends, whereas thechannels of wall-flow substrates have one end capped, wherein thecapping occurs on opposite ends of adjacent channels in an alternatingpattern. Capping alternating ends of channels prevents the gas enteringthe inlet face of the substrate from flowing straight through thechannel and existing. Instead, the exhaust gas enters the front of thesubstrate and travels into about half of the channels where it is forcedthrough the channel walls prior to entering the second half of thechannels and exiting the back face of the substrate.

The substrate wall has a porosity and pore size that is gas permeable,but traps a major portion of the particulate matter, such as soot, fromthe gas as the gas passes through the wall. Preferred wall-flowsubstrates are high efficiency filters. Wall flow filters for use withthe present invention preferably have an efficiency of least 70%, atleast about 75%, at least about 80%, or at least about 90%. In certainembodiments, the efficiency will be from about 75 to about 99%, about 75to about 90%, about 80 to about 90%, or about 85 to about 95%. Here,efficiency is relative to soot and other similarly sized particles andto particulate concentrations typically found in conventional dieselexhaust gas. For example, particulates in diesel exhaust can range insize from 0.05 microns to 2.5 microns. Thus, the efficiency can be basedon this range or a sub-range, such as 0.1 to 0.25 microns, 0.25 to 1.25microns, or 1.25 to 2.5 microns.

Porosity is a measure of the percentage of void space in a poroussubstrate and is related to backpressure in an exhaust system:generally, the lower the porosity, the higher the backpressure.Preferably, the porous substrate has a porosity of about 30 to about80%, for example about 40 to about 75%, about 40 to about 65%, or fromabout 50 to about 60%.

The pore interconnectivity, measured as a percentage of the substrate'stotal void volume, is the degree to which pores, void, and/or channels,are joined to form continuous paths through a porous substrate, i.e.,from the inlet face to the outlet face. In contrast to poreinterconnectivity is the sum of closed pore volume and the volume ofpores that have a conduit to only one of the surfaces of the substrate.Preferably, the porous substrate has a pore interconnectivity volume ofat least about 30%, more preferably at least about 40%.

The mean pore size of the porous substrate is also important forfiltration. Mean pore size can be determined by any acceptable means,including by mercury porosimetry. The mean pore size of the poroussubstrate should be of a high enough value to promote low backpressure,while providing an adequate efficiency by either the substrate per se,by promotion of a soot cake layer on the surface of the substrate, orcombination of both. Preferred porous substrates have a mean pore sizeof about 10 to about 40 μm, for example about 20 to about 30 μm, about10 to about 25 μm, about 10 to about 20 μm, about 20 to about 25 μm,about 10 to about 15 μm, and about 15 to about 20 μm.

In general, the production of an extruded solid body containing themetal promoted zeolite catalyst involves blending the zeolite and thetransition metal (either separately or together as a metal-exchangedzeolite), a binder, an optional organic viscosity-enhancing compoundinto an homogeneous paste which is then added to a binder/matrixcomponent or a precursor thereof and optionally one or more ofstabilized ceria, and inorganic fibers. The blend is compacted in amixing or kneading apparatus or an extruder. The mixtures have organicadditives such as binders, pore formers, plasticizers, surfactants,lubricants, dispersants as processing aids to enhance wetting andtherefore produce a uniform batch. The resulting plastic material isthen molded, in particular using an extrusion press or an extruderincluding an extrusion die, and the resulting moldings are dried andcalcined. The organic additives are “burnt out” during calcinations ofthe extruded solid body. A metal-promoted zeolite catalyst may also bewashcoated or otherwise applied to the extruded solid body as one ormore sub-layers that reside on the surface or penetrate wholly or partlyinto the extruded solid body. Alternatively, a metal-promoted zeolitecan be added to the paste prior to extrusion.

Extruded solid bodies containing metal-promoted zeolites according tothe present invention generally comprise a unitary structure in the formof a honeycomb having uniform-sized and parallel channels extending froma first end to a second end thereof. Channel walls defining the channelsare porous. Typically, an external “skin” surrounds a plurality of thechannels of the extruded solid body. The extruded solid body can beformed from any desired cross section, such as circular, square or oval.Individual channels in the plurality of channels can be square,triangular, hexagonal, circular etc. Channels at a first, upstream endcan be blocked, e.g. with a suitable ceramic cement, and channels notblocked at the first, upstream end can also be blocked at a second,downstream end to form a wall-flow filter. Typically, the arrangement ofthe blocked channels at the first, upstream end resembles achecker-board with a similar arrangement of blocked and open downstreamchannel ends.

The binder/matrix component is preferably selected from the groupconsisting of cordierite, nitrides, carbides, borides, intermetallics,lithium aluminosilicate, a spinel, an optionally doped alumina, a silicasource, titania, zirconia, titania-zirconia, zircon and mixtures of anytwo or more thereof. The paste can optionally contain reinforcinginorganic fibers selected from the group consisting of carbon fibers,glass fibers, metal fibers, boron fibers, alumina fibers, silica fibers,silica-alumina fibers, silicon carbide fibers, potassium titanatefibers, aluminum borate fibers and ceramic fibers.

The alumina binder/matrix component is preferably gamma alumina, but canbe any other transition alumina, i.e., alpha alumina, beta alumina, chialumina, eta alumina, rho alumina, kappa alumina, theta alumina, deltaalumina, lanthanum beta alumina and mixtures of any two or more suchtransition aluminas. It is preferred that the alumina is doped with atleast one non-aluminum element to increase the thermal stability of thealumina. Suitable alumina dopants include silicon, zirconium, barium,lanthanides and mixtures of any two or more thereof. Suitable lanthanidedopants include La, Ce, Nd, Pr, Gd and mixtures of any two or morethereof.

Sources of silica can include a silica sol, quartz, fused or amorphoussilica, sodium silicate, an amorphous aluminosilicate, an alkoxysilane,a silicone resin binder such as methylphenyl silicone resin, a clay,talc or a mixture of any two or more thereof. Of this list, the silicacan be SiO₂ as such, feldspar, mullite, silica-alumina, silica-magnesia,silica-zirconia, silica-thoria, silica-berylia, silica-titania, ternarysilica-alumina-zirconia, ternary silica-alumina-magnesia,ternary-silica-magnesia-zirconia, ternary silica-alumina-thoria andmixtures of any two or more thereof.

Preferably, the metal-promoted zeolite is dispersed throughout, andpreferably evenly throughout, the entire extruded catalyst body.

Where any of the above extruded solid bodies are made into a wall-flowfilter, the porosity of the wall-flow filter can be from 30-80%, such asfrom 40-70%. Porosity and pore volume and pore radius can be measurede.g. using mercury intrusion porosimetry.

The metal-promoted zeolite catalyst described herein can promote thereaction of a reductant, preferably ammonia, with nitrogen oxides toselectively form elemental nitrogen (N₂) and water (H₂O). Thus, in oneembodiment, the catalyst can be formulated to favor the reduction ofnitrogen oxides with a reductant (i.e., an SCR catalyst). Examples ofsuch reductants include hydrocarbons (e.g., C3-C6 hydrocarbons) andnitrogenous reductants such as ammonia and ammonia hydrazine or anysuitable ammonia precursor, such as urea ((NH₂)₂CO), ammonium carbonate,ammonium carbamate, ammonium hydrogen carbonate or ammonium formate. Forexample, the SCR process of the present method can result in a NO_(x)conversion of at least 75%, preferably at least 80%, and more preferablyat least 90%. The NO conversion can be represented as the relative ratioof NO concentration at the SCR inlet (unpurified exhaust gas) comparedto the NO concentration at the SCR outlet and/or ASC outlet (purifiedexhaust gas). Preferably, the inlet NO concentration and outlet NOconcentration have a ratio of greater than about 4, greater than about5, or greater than about 10 over a broad temperature range (e.g., about150-700° C., about 200-350° C., about 350-550° C., or about 450-550°C.). Likewise, the NO₂ conversion can be represented as the relativeratio of NO₂ concentration at the SCR inlet (unpurified exhaust gas)compared to the NO₂ concentration at the SCR outlet and/or ASC outlet(purified exhaust gas). Preferably, the inlet NO₂ concentration andoutlet NO₂ concentration have a ratio of greater than about 4, greaterthan about 5, or greater than about 10 over a broad temperature range(e.g., about 150-700° C., about 200-350° C., about 350-550° C., or about450-550° C.).

Importantly, the use of low SAR, small pore zeolites according to thepresent invention generates very low amounts of N₂O compared toconventional zeolite catalysts. That is, the SCR process of the presentmethod can result in low N₂O generation based on NO and/or NO₂ at theSCR inlet. For example, the relative ratio of inlet NO concentration atthe SCR catalyst compared to outlet N₂O concentration after the SCRand/or ASC catalyst is greater than about 50, greater than about 80, orgreater than about 100 over a broad temperature range (e.g., about150-700° C., about 200-350° C., about 350-550° C., or about 450-550°C.). In another example, the relative ratio of inlet NO₂ concentrationat the SCR catalyst compared to outlet N₂O concentration after the SCRand/or ASC catalyst is greater than about 50, greater than about 80, orgreater than about 100 over a broad temperature range (e.g., about150-700° C., about 200-350° C., about 350-550° C., or about 450-550°C.).

The metal-promoted zeolite catalyst described herein can also promotethe oxidation of ammonia. Thus, in another embodiment, the catalyst canbe formulated to favor the oxidation of ammonia with oxygen,particularly a concentrations of ammonia typically encountereddownstream of an SCR catalyst (e.g., ammonia oxidation (AMOX) catalyst,such as an ammonia slip catalyst (ASC)). In certain embodiments, themetal-promoted zeolite catalyst is disposed as a top layer over anoxidative under-layer, wherein the under-layer comprises a platinumgroup metal (PGM) catalyst or a non-PGM catalyst. Preferably, thecatalyst component in the underlayer is disposed on a high surface areasupport, including but not limited to alumina.

In yet another embodiment, an SCR and AMOX operations are performed inseries, wherein both processes utilize a catalyst comprising themetal-promoted zeolite described herein, and wherein the SCR processoccurs upstream of the AMOX process. For example, an SCR formulation ofthe catalyst can be disposed on the inlet side of a filter and an AMOXformulation of the catalyst can be disposed on the outlet side of thefilter.

Accordingly, provided is a method for the reduction of NO_(x) compoundsor oxidation of NH₃ in a gas, which comprises contacting the gas with acatalyst composition described herein for the catalytic reduction ofNO_(x) compounds for a time sufficient to reduce the level of NO_(x)compounds and/or NH₃ in the gas. In certain embodiments, provided is acatalyst article having an ammonia slip catalyst disposed downstream ofa selective catalytic reduction (SCR) catalyst. In such embodiments, theammonia slip catalyst oxidizes at least a portion of any nitrogenousreductant that is not consumed by the selective catalytic reductionprocess. For example, in certain embodiments, the ammonia slip catalystis disposed on the outlet side of a wall flow filter and an SCR catalystis disposed on the upstream side of a filter. In certain otherembodiments, the ammonia slip catalyst is disposed on the downstream endof a flow-through substrate and an SCR catalyst is disposed on theupstream end of the flow-through substrate. In other embodiments, theammonia slip catalyst and SCR catalyst are disposed on separate brickswithin the exhaust system. These separate bricks can be adjacent to, andin contact with, each other or separated by a specific distance,provided that they are in fluid communication with each other andprovided that the SCR catalyst brick is disposed upstream of the ammoniaslip catalyst brick.

In certain embodiments, the SCR and/or AMOX process is performed at atemperature of at least 100° C. In another embodiment, the process(es)occur at a temperature from about 150° C. to about 750° C. In aparticular embodiment, the temperature range is from about 175 to about550° C. In another embodiment, the temperature range is from 175 to 400°C. In yet another embodiment, the temperature range is 450 to 900° C.,preferably 500 to 750° C., 500 to 650° C., 450 to 550° C., or 650 to850° C. Embodiments utilizing temperatures greater than 450° C. areparticularly useful for treating exhaust gases from a heavy and lightduty diesel engine that is equipped with an exhaust system comprising(optionally catalyzed) diesel particulate filters which are regeneratedactively, e.g. by injecting hydrocarbon into the exhaust system upstreamof the filter, wherein the zeolite catalyst for use in the presentinvention is located downstream of the filter.

According to another aspect of the invention, provided is a method forthe reduction of NO_(x) compounds and/or oxidation of NH₃ in a gas,which comprises contacting the gas with a catalyst described herein fora time sufficient to reduce the level of NO_(x) compounds in the gas.Methods of the present invention may comprise one or more of thefollowing steps: (a) accumulating and/or combusting soot that is incontact with the inlet of a catalytic filter; (b) introducing anitrogenous reducing agent into the exhaust gas stream prior tocontacting the catalytic filter, preferably with no interveningcatalytic steps involving the treatment of NO_(x) and the reductant; (c)generating NH₃ over a NO_(x) adsorber catalyst or lean NO_(x) trap, andpreferably using such NH₃ as a reductant in a downstream SCR reaction;(d) contacting the exhaust gas stream with a DOC to oxidize hydrocarbonbased soluble organic fraction (SOF) and/or carbon monoxide into CO₂,and/or oxidize NO into NO₂, which in turn, may be used to oxidizeparticulate matter in particulate filter; and/or reduce the particulatematter (PM) in the exhaust gas; (e) contacting the exhaust gas with oneor more flow-through SCR catalyst device(s) in the presence of areducing agent to reduce the NOx concentration in the exhaust gas; and(f) contacting the exhaust gas with an ammonia slip catalyst, preferablydownstream of the SCR catalyst to oxidize most, if not all, of theammonia prior to emitting the exhaust gas into the atmosphere or passingthe exhaust gas through a recirculation loop prior to exhaust gasentering/re-entering the engine.

In another embodiment, all or at least a portion of the nitrogen-basedreductant, particularly NH₃, for consumption in the SCR process can besupplied by a NO_(x) adsorber catalyst (NAC), a lean NO_(x) trap (LNT),or a NO_(x) storage/reduction catalyst (NSRC), disposed upstream of theSCR catalyst, e.g., a SCR catalyst of the present invention disposed ona wall-flow filter. NAC components useful in the present inventioninclude a catalyst combination of a basic material (such as alkalimetal, alkaline earth metal or a rare earth metal, including oxides ofalkali metals, oxides of alkaline earth metals, and combinationsthereof), and a precious metal (such as platinum), and optionally areduction catalyst component, such as rhodium. Specific types of basicmaterial useful in the NAC include cesium oxide, potassium oxide,magnesium oxide, sodium oxide, calcium oxide, strontium oxide, bariumoxide, and combinations thereof. The precious metal is preferablypresent at about 10 to about 200 g/ft³, such as 20 to 60 g/ft³.Alternatively, the precious metal of the catalyst is characterized bythe average concentration which may be from about 40 to about 100grams/ft³.

Under certain conditions, during the periodically rich regenerationevents, NH₃ may be generated over a NO_(x) adsorber catalyst. The SCRcatalyst downstream of the NO_(x) adsorber catalyst may improve theoverall system NO_(x) reduction efficiency. In the combined system, theSCR catalyst is capable of storing the released NH₃ from the NACcatalyst during rich regeneration events and utilizes the stored NH₃ toselectively reduce some or all of the NO_(x) that slips through the NACcatalyst during the normal lean operation conditions.

The method for treating exhaust gas as described herein can be performedon an exhaust gas derived from a combustion process, such as from aninternal combustion engine (whether mobile or stationary), a gas turbineand coal or oil fired power plants. The method may also be used to treatgas from industrial processes such as refining, from refinery heatersand boilers, furnaces, the chemical processing industry, coke ovens,municipal waste plants and incinerators, etc. In a particularembodiment, the method is used for treating exhaust gas from a vehicularlean burn internal combustion engine, such as a diesel engine, alean-burn gasoline engine or an engine powered by liquid petroleum gasor natural gas.

In certain aspects, the invention is a system for treating exhaust gasgenerated by combustion process, such as from an internal combustionengine (whether mobile or stationary), a gas turbine, coal or oil firedpower plants, and the like. Such systems include a catalytic articlecomprising the metal-promoted zeolite described herein and at least oneadditional component for treating the exhaust gas, wherein the catalyticarticle and at least one additional component are designed to functionas a coherent unit.

In certain embodiments, the system comprises a catalytic articlecomprising a metal-promoted zeolite described herein, a conduit fordirecting a flowing exhaust gas, a source of nitrogenous reductantdisposed upstream of the catalytic article. The system can include acontroller for the metering the nitrogenous reductant into the flowingexhaust gas only when it is determined that the zeolite catalyst iscapable of catalyzing NO_(x) reduction at or above a desired efficiency,such as at above 100° C., above 150° C. or above 175° C. The metering ofthe nitrogenous reductant can be arranged such that 60% to 200% oftheoretical ammonia is present in exhaust gas entering the SCR catalystcalculated at 1:1 NH₃/NO and 4:3 NH₃/NO₂.

In another embodiment, the system comprises an oxidation catalyst (e.g.,a diesel oxidation catalyst (DOC)) for oxidizing nitrogen monoxide inthe exhaust gas to nitrogen dioxide can be located upstream of a pointof metering the nitrogenous reductant into the exhaust gas. In oneembodiment, the oxidation catalyst is adapted to yield a gas streamentering the SCR zeolite catalyst having a ratio of NO to NO₂ of fromabout 4:1 to about 1:3 by volume, e.g. at an exhaust gas temperature atoxidation catalyst inlet of 250° C. to 450° C. The oxidation catalystcan include at least one platinum group metal (or some combination ofthese), such as platinum, palladium, or rhodium, coated on aflow-through monolith substrate. In one embodiment, the at least oneplatinum group metal is platinum, palladium or a combination of bothplatinum and palladium. The platinum group metal can be supported on ahigh surface area washcoat component such as alumina, a zeolite such asan aluminosilicate zeolite, silica, non-zeolite silica alumina, ceria,zirconia, titania or a mixed or composite oxide containing both ceriaand zirconia.

In a further embodiment, a suitable filter substrate is located betweenthe oxidation catalyst and the SCR catalyst. Filter substrates can beselected from any of those mentioned above, e.g. wall flow filters.Where the filter is catalyzed, e.g. with an oxidation catalyst of thekind discussed above, preferably the point of metering nitrogenousreductant is located between the filter and the zeolite catalyst.Alternatively, if the filter is un-catalyzed, the means for meteringnitrogenous reductant can be located between the oxidation catalyst andthe filter.

Turning to FIG. 2, shown is an embodiment of the invention comprising anSCR and/or ASC catalyst 10, an exhaust gas 20, a purified gas 22, and adirection of flow through the SCR and/or ASC catalyst 30. In certainembodiments, the exhaust gas 20 has an inlet concentration of NO and/orNO₂ and the purified gas 22 has an outlet concentration of NO and/or NO₂that is less than the inlet concentration. The purified gas 22 also hasan outlet concentration of N₂O that is less than the inlet concentrationof NO and/or NO₂.

EXAMPLES Example 1

A series of zeolites with two types of framework structures wereevaluated at different silica to alumina ratios (SAR), CHA (SAR=26, 17,13), ERI (SAR=13 and 7). The copper loading on these catalysts weremaintain the same at 3 wt. %. Exchanged zeolites were calcined to 500°C. before being coated onto 400 cpsi/4.5 mil ceramic substrates. Coresamples (1″×2″) were evaluated at a GHSV=50,000 h⁻¹ in gas consisting of350 ppm NH₃, 350 ppm NO, 14% O₂, 4.6% H₂O, 5% CO2 in N₂. Inlet andoutlet gas compositions were monitored by FTIR to determine theconversion efficiency of NO and the formation of N₂O.

The steady state NO_(x) conversions as a function of temperature overthe catalysts are summarized in FIG. 1 as solid lines. Although all thecatalysts show high and comparable NO_(x) conversion at temperaturesbelow 350° C. regardless of the SAR of the zeolite supports, a cleartrend is seen that the NO_(x) conversion at 550° C. increases with thedecrease of the SAR. Additionally, the N₂O formation, plotted in FIG. 1as dash curves, also show a clear dependence on the SARs of the zeolitesupports; the low SAR samples regardless of framework type consistentlyresult in lower N₂O formation.

What is claimed is:
 1. A method for reducing N₂O emissions in an exhaustgas comprising contacting an exhaust gas containing NH₃ and an inlet NOconcentration with an SCR catalyst composition containing small porezeolite having an SAR of about 3 to about 15 and having about 1-5 wt. %of an exchanged transition metal to produce a purified gas containing anoutlet NO concentration and an outlet N₂O concentration, a. wherein theinlet NO concentration and outlet NO concentration have a relative ratioof>about 4, and b. wherein the inlet NO concentration and outlet N₂Oconcentration have a relative ratio of>about
 50. 2. The method of claim1, wherein the inlet NO concentration and outlet NO concentration have arelative ratio of>about
 5. 3. The method of claim 1, wherein the inletNO concentration and outlet NO concentration have a relative ratioof>about
 10. 4. The method of claim 1, wherein the inlet NOconcentration and outlet N₂O concentration have a relative ratioof>about
 80. 5. The method of claim 4, wherein the inlet NOconcentration and outlet N₂O concentration have a relative ratioof>about
 100. 6. The method of claim 1, wherein the transition metal iscopper.
 7. The method of claim 1, wherein the small pore zeolite has aSAR of about 5 to about
 9. 8. The method of claim 1, wherein the smallpore zeolite has a SAR of about 10 to about
 15. 9. The method of claim1, wherein the small pore zeolite has a framework selected from AEI,AFT, AFX, CHA, DDR, ERI, KFI, LEV, RHO, and UEI.
 10. The method of claim1, wherein the small pore zeolite has an AEI framework.
 11. The methodof claim 1, wherein the small pore zeolite has a CHA framework.
 12. Themethod of claim 1, wherein the small pore zeolite has an ERI framework.13. The method of claim 1, wherein the NO and NO₂ are present in a ratioof about 4:1 to about 1:3 by volume.
 14. The method of claim 1, whereinthe NH₃ is present in exhaust gas entering the SCR catalyst at an NH₃/NOratio of about 0.5:1 to about 1:2 and an NH₃/NO₂ ratio of about 1:1 toabout 6:3.
 15. A system for treating an exhaust gas comprising, inseries and in fluid communication, a diesel oxidation catalyst, a sourceof nitrogen-based reductant, and an SCR catalyst, wherein the SCRcatalyst comprises a small pore zeolite having an SAR of about 3 toabout 15 and having about 1-5 wt. % of an exchanged transition metal,and wherein the SCR catalyst is coated on a honeycomb wall-flow filteror flow-through monolith or is an extruded honeycomb body.